1. Field of the Invention
The present invention relates generally to printed circuit boards. More particularly, the invention relates to techniques for mounting integrated circuits and other electrical components on a substrate and electrically interconnecting the devices.
2. Description of the Related Art
Most electronics equipment manufactured today includes integrated circuits ("IC's") and other electrical components mounted on printed circuit boards ("PCB"). Printed circuit boards (alternatively called "printed wiring boards") allow the IC's to be interconnected in a cost effective and reliable manner.
Referring now to FIG. 1, a conventional PCB 20 is shown to include a substrate 21 and a number of IC's 24, 26, 28, 30, 32. It should be recognized that the substrate is shown disproportionately thick for ease of understanding. The substrate is constructed of plastic, ceramic, or other suitable material. Further, the substrate may include multiple layers of conductive sheets. In accordance with known manufacturing techniques, the conducting layers are sandwiched together, with each conductive layer separated by an insulating layer. Electrical connections between the components mounted on the top of the substrate are made via conductive "traces" on the top surface 22 of the PCB (an exemplary trace 27 is shown FIG. 1). Other connections are made by traces on one or more of the conductive layers included within the substrate.
Referring now to FIG. 2, two exemplary IC's 24, 26 are shown in a cross-sectional view bonded to the top surface 22 of substrate 21 using a conductive epoxy 25 or other conductive adhesive. Electrical connections between each IC 24, 26 and the substrate are made by way of wire bonds 34a, 34b, 34c, and 34d. Wire bonds 34a-34d electrically couple contacts 29 on the top surface of each IC to contacts 23a, 23b, 23c, and 23d (referred to as "pads" and also shown in FIG. 1) disposed on the top surface 22 of the substrate 21. The wire bonds 34 are thin, curved loops of wire, typically made of gold or aluminum.
Referring still to FIG. 2, IC 24 electrically couples to IC 26 by way of a trace on conductive layer 36. Pads 23b and 23c electrically connect to conductive layer 36 by way of conducting channels 38 and 40 included within substrate 21. Conducting channels 38, 40 couple to layer 36 at points 37 and 39, respectively. To prevent conducting channels 38, 40 from electrically coupling to other conducting layers between pads 23b, 23c and layer 36, conducting channels 38, 40 are contained within an insulated hole (not shown) in the substrate, commonly referred to as a "via" hole or simply "via." Thus, an electrical signal passing from IC 24 to IC 26 passes through wire bond 34b, pad 23b, conductive channel 38, layer 36 between points 37 and 39, conducting channel 40, pad 23c, and wire bond 34c.
Several disadvantages of current PCB technology are illustrated with respect to FIGS. 1 and 2. Referring to FIG. 1, a substrate has a limited surface area in which IC's and other electrical components can be mounted. Thus, there is a limit to the size and number of IC's that can be mounted on a given PCB. The number of IC's that are mounted on a given substrate is referred to as "packaging density." The size of a given substrate, and in particular its surface area, traditionally has placed an upper limit on packaging density.
For many types of electronic devices, the ability to provide a high packaging density is extremely important. Pacemakers and defibrillators, for example, include charging circuitry for charging capacitors to deliver electrical shocks to the heart, amplifier circuits for amplifying electrical signals produced by the heart, microprocessor circuitry for controlling operation of the device, as well as other components. It is now commonplace for pacemakers and defibrillators to be implanted inside the body. As implanted device technology has progressed, the requirements and capabilities of such devices have increased dramatically. Many of the technological advancements in implanted device technology have required the addition of new components to the device. Future advancement in implantable pacemaker and defibrillator technology may, and probably will, require still more electronic components to be included within the implanted device.
To minimize patient discomfort, it is desirable to make an implantable device as small as possible. Thus, implantable device manufacturers are faced with two conflicting design criteria. It is desired, on one hand, to include more components in the implantable device. At the same time, it is desirable to make the implantable devices even smaller so as to increase patient comfort. Moreover, being able to include more components in a smaller volume, i.e., increasing packaging density, has long been a design goal of all implantable device manufacturers. Likewise, for many other types of electronic equipment, such as laptop computers, digital watches, and hearing aids, increasing packaging density also is desirable.
Another disadvantage that must be endured with currently available printed circuit board designs is illustrated with reference to FIG. 2. As explained above, an electrical signal (i.e., electrical current) from IC 24 must pass through a length of conductive materials including wire bonds 34b and 34c, conducting channels 38 and 40 and layer 36 between points 37 and 39. It is known that an electrical conductor has a parameter referred to as resistance or more broadly, impedance. The impedance of a conductor is proportional to the length of the conductor--a longer conductor has a higher impedance than a shorter conductor for a given cross-sectional area. The impedance of a conductor causes a drop in voltage between opposite ends of the conductor as a result of the current that flows through the conductor. The resulting voltage drop increases as the length of the conductor increases. With reference to FIG. 2, a signal produced by IC 24 may have, for example, a voltage of 3 volts as the signal leaves IC 24 and enters wire bond 34b. However, because of the length of the conductor, comprised of wire bonds 34b and 34c, conducting channels 38 and 40 and layer 36, the voltage of the signal as it leaves wire bond 34c and enters IC 26 (the opposite end of the conductor) will be less than 3 volts. The magnitude of the voltage of the signal at IC 26 depends on the length of the conducting path through which the signal passes between IC 24 and IC 26. In some situations, as explained below, this voltage drop may be undesirable.
Referring again to the pacemaker and defibrillator example, IC 26 may be a capacitor, and IC 24 may be a charging circuit for charging the capacitor 26. The electrical charge stored in the capacitor 26 is used to deliver a shock to the heart according to known medical protocols. The charging circuit 24 generates and provides electrical charging current to the capacitor via wire bond 34b, conducting channel 38, layer 36, conducting channel 40, and wire bond 34c. The time it takes to charge a capacitor is a direct function of the efficiency of the charging process--a low efficiency charging process requires more time to charge a capacitor than a highly efficient charging process. The charging efficiency is less than 100% for the circuit of FIG. 2 because, among other reasons, electrical energy is lost as a result of the voltage drop along the conducting pathway between the charging circuit 24 and capacitor 26. Thus, it will take more time to charge capacitor 26 than if there was no voltage drop along the conducting pathway between charging circuit 24 and capacitor 26. Moreover, the length of the conducting pathway directly effects the charging time. For this reason and others, reducing the impedance of a conductive path often is desirable.
It would thus be desirable to have printed circuit board technology that allows for higher packaging density. Such a circuit board would allow for more electrical components to be included in a smaller volume than previously possible and thus would especially benefit those devices for which a smaller size is desirable. Further, printed circuit board technology that includes lower impedance conductive pathways also is desired. Despite the advancements made in printed circuit board technology, to date there is still room for improvement in these areas.